METALS Grain growth mechanisms in ultrafine-grained steel: an electron backscatter diffraction and in situ TEM study Laura Ahmels 1 , Ankush Kashiwar 1,2 , Torsten Scherer 2,3 , Christian Ku ¨bel 2,3 , and Enrico Bruder 1, * 1 Division Physical Metallurgy, Materials Science Department, TU Darmstadt, Alarich-Weiss-Str. 2, 64287 Darmstadt, Germany 2 Institute of Nanotechnology, Karlsruhe Institute of Technology, Hermann-von-Helmholtz Platz 1, 767344 Eggenstein, Germany 3 Karlsruhe Nano Micro Facility, Karlsruhe Institute of Technology, Hermannsschlacht Platz 1, 767344 Eggenstein, Germany Received: 21 February 2019 Accepted: 8 April 2019 Ó Springer Science+Business Media, LLC, part of Springer Nature 2019 ABSTRACT In this work, the thermal stability of the strongly elongated and textured ultrafine-grained microstructure of a high-strength low-alloy steel processed by linear flow splitting is investigated. The annealing behavior is studied for a- and c-fiber orientations, which are dominant in the rolling type texture of the material and are known for their differences in stored energy in conventionally cold rolled steels. Electron backscatter diffraction is used to assess contributions from curvature-driven boundary migration while the contribution of strain- induced migration is investigated by comparing the annealing behavior of prerecovered to non-prerecovered samples. The exact nature of the coarsening process is studied using in situ TEM heat treatments and complementary ACOM analysis. The results show that the observed preferred growth of a-fiber grains can be fully described by curvature-driven grain boundary migration and that there is no indication for the relevance of gradients in dislocation density. Introduction A common characteristic of most ultrafine-grained (UFG) metals is an intrinsic instability at elevated tem- peratures and in some cases even at room temperature [1, 2] caused by the high defect density that results from severe plastic deformation (SPD) processing [3]. To utilize the full potential of UFG materials for structural or functional applications, the microstructure and related properties have to be optimized by subsequent annealing [4–7] and need to be stable during service. Even if limited microstructural and thus property alterations are tolerable to some extent, they need to be predictable. Understanding the active mechanisms and driving forces that promote thermally activated coars- ening processes is therefore not just an interesting topic for fundamental research but also of substantial tech- nological relevance. UFG materials take a special role with respect to thermal stability and annealing phenomena. Being Address correspondence to E-mail: e.bruder@phm.tu-darmstadt.de https://doi.org/10.1007/s10853-019-03611-8 J Mater Sci Metals